Pedals – If you purchase a bike at a dealer most likely the bike will not come with pedals. While pedals are a personal preference there are some considerations. There are different standard that work with different cleat designs and weight. The lighter the weight the higher the cost. My suggest would be Shimano and their SPD-SL line of pedals. I suggest the Shimano 105 PD-5700 as a Starting point.
Alternatively some folks dislike a one-sided pedal and opt for mountain bike pedals to take advantage of the dual entry design. Meaning the pedal has two identical sides. My suggestion there would be the Shimano XT series pedal.
Regardless of which style pedal you chose you will need the appropriate cleat for your shoes
Helmet – Simple fact is helmet is probably the most important purchase to go with your bike. Fortunately there are many good choices that wont break the bank. I suggest going to your local bike shop and see what fits and what looks good on you and buy from them. All helmets do not fit everyone the same. That said If you absolutely must buy a helmet online look at the reviews and save yourself some money and try to find a model that is from a previous year. You can save yourself a substantial amount of money. Also keep in mind helmets do not last forever and they should be replaced every few years (5 max) or after taking a hit.